1. Field of the Invention
This invention relates to a plural-dimensional displacement measuring system for an apparatus (such as a semiconductor or LCD manufacturing apparatus) having a bed, a first stage movable in a first direction (say Y) relative to the bed and a second stage movable in a second direction (say X), generally at right angles to the first Y direction, relative to the first stage.
2. Description of the Prior Art
In such an apparatus, the first and second stages are typically driven by respective electric motors that may be fitted with encoders in order to obtain a rough measure of the position of the second stage relative to the bed. However, semiconductor and LCD manufacture requires very high precision positioning, and in order to achieve this it is known to employ interferometers to measure the position, or displacement, of the second stage relative to the bed.
The invention relates, more particularly, to a measuring system employing a first interferometer system for measuring linear displacement generally in the first Y direction between a first reference reflector and a first target reflector fixed relative to the first stage and producing a corresponding first output signal, a second interferometer system for measuring linear displacement generally in the second X direction between a second reference reflector and a second target reflector fixed relative to the second stage and producing a corresponding second output signal, and means for processing the output signals to determine linear displacement of the second stage relative to the bed in the first and second directions,
Such a measuring system is known. The first interferometer system includes a first interferometer head, having a first helium-neon (He-Ne) laser and the first reference reflector, mounted on the bed, and the second interferometer system includes a second interferometer head, having a second helium-neon (He-Ne) laser and the second reference reflector, mounted on the first stage. The requirement of mounting the second interferometer head, including its laser, on the first stage is a disadvantage, in view of the space required, the weight and the need to retrieve the second output signal from the second interferometer system.